1. Field of the Invention
This invention relates to a security circuit for protecting data written into a ROM (Read Only Memory) built in a one-chip microcomputer, for example. 2. Description of the Related Art
A security circuit for protecting data stored in a memory built into a microcomputer has been developed. For example, the security circuit includes one type of security circuit which makes it impossible to read out data when it is operated and another type of security circuit which permits only the user who has set the security data to read out classified data, such as programs stored in the memory, and inhibits the other users from reading data, as is disclosed in Japanese Patent Disclosure No. H.4-76749 filed by the same applicant.
However, in either type of the above conventional security circuits, data cannot be read out when a user other than the user who has set the security data has accessed the memory. The other user quickly recognizes that the security is set in the memory.
Generally, if security data is input a number of times corresponding to the number of bits of security data set by the security circuit while changing the contents of the input key data, the security data may coincide with the input key data and will be released or destroyed. Therefore, if it is understood that the security is set in the memory, it is only a matter of time before data, stored in the memory, is read out by the other user and becomes difficult to protect the data stored in the memory.